1. Field of Invention
The present invention relates to a variable compression ratio apparatus. More particularly, the present invention relates to a variable compression ratio apparatus which changes compression ratio of an air-fuel mixture in a combustion chamber according to a driving condition of an engine.
2. Description of Related Art
Generally, thermal efficiency of combustion engines increases as the compression ratio thereof increases, and if ignition timing is advanced to some degree, thermal efficiency of spark-ignition engines increases. However, if the ignition timing of the spark-ignition engines is advanced at a high compression ratio, abnormal combustion may occur and the engine may be damaged. Thus, the ignition timing cannot be excessively advanced and accordingly engine output may deteriorate.
A variable compression ratio (VCR) apparatus changes the compression ratio of an air-fuel mixture according to a driving condition of the engine. The variable compression ratio apparatus raises the compression ratio of the air-fuel mixture at a low-load condition of the engine in order to improve fuel mileage. On the contrary, the variable compression ratio apparatus lowers the compression ratio of the air-fuel mixture at a high-load condition of the engine in order to prevent occurrence of knocking and improve engine output.
According to a conventional variable compression ratio apparatus, a variable compression ratio is achieved by changing a length of a connecting rod which connects a piston to a crankshaft. Such types of a variable compression ratio apparatus include a plurality of links connecting a piston with the crankshaft, and combustion force is directly transmitted to the links. So, durability of the links deteriorates.
It becomes known to a person skilled in the art through various experimental results conducted to a conventional variable compression ratio apparatus that operation reliability is high in a case that a distance between a crank pin and a piston pin is changed by using an eccentric bearing. If hydraulic pressure, however, is used for rotating the eccentric bearing, a rotating angle of the eccentric bearing in each cylinder or hydraulic pressure applied to each cylinder is different. So, a compression ratio in a cylinder is different from that in another cylinder and a time required for changing the compression ratio according to the driving condition of the engine is varied in each cylinder.
Since links for rotating the eccentric bearing can be escaped horizontally, additional devices for limiting movement trace of the links should be provided at the connecting rod or the links. Therefore, structures of the connecting rod or the links may become complex and moving mass may increase.
Meanwhile, many inventors have been developing that the movement trace of the links is limited by using balance weights provided at a crankshaft without change in a structure of the connecting rod or the links. In this case, length of the link may become longer such that a portion of the link is always positioned in movement trace of the balance weights, and size of the balance weights may become larger. In addition, a portion of the balance weights must be trimmed so as to prevent interference between movement traces of the links and the balance weights.
The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.